In all domains of Medical and Scientific X-Ray imaging, replacement of legacy film and analog video processing by a fully digital workflow is underway. There are several drivers for this conversion, including increased operational efficiency and faster work flow (patient throughput), adoption of digital information workflow and archiving infrastructure in the hospitals and electronic sharing of results for peer reviews, the advanced digital image processing capabilities, and the lower X-ray dose and real-time availability that digital X-ray imaging solutions enable.

Digital X-Ray Technology Workflow Benefits:

  • increased operational efficiency resulting in higher patient comfort;
  • the adoption of digital information workflow and archival infrastructure;
  • opportunity for digital communication and collaboration with peers at remote sites
  • advanced digital image processing capabilities;
  • and lower X-Ray dose with real-time image availability

There are four primary digital X-Ray technologies in use today, these include:

  • Computed Radiography – uses phosphor imaging plates that are converted to digital format in a separate scanning operation (in a similar manner to traditional film-scanning) and cannot be used for real-time X-Ray imaging such as endovascular procedures.
  • Image Intensified CCD-cameras (II-CCD) – in which the x-ray signal is converted to electrical charges (electrons), which are multiplied and converted to visible light photons inside a high voltage glass vacuum tube. The visible light is imaged onto a CCD-camera to generate real-time images at low x-ray dose levels, e.g. to act as the surgeon’s eyes during minimally invasive procedures. Although recognized for their excellent for low-dose image quality and cost-effectiveness, II-CCD cameras offer significant drawbacks in terms of physical size, inability to support challenging projections, vulnerability and shorter intervals between calibrations.
  • Flat Direct Detectors – Direct conversion uses either amorphous selenium (aSe), cadmium telluride (CdTe) or mercuric iodine (HgI2) layers to convert X-Ray photons directly to electrons for immediate image capture. An important drawback of selenium is the instability and mechanical vulnerability of the material over time and under normal transport temperature conditions, as well as the environmental impact during production and repair or disposal. Amorphous selenium is also suffering from ‘image lag’, a memory effect where information from previous images is retained during next captures, rendering the technology unsuitable for real-time imaging applications.
  • Flat Indirect Detectors – As the most common method, indirect detection uses scintillators (such as Gadolinium Oxides or Cesium-Iodides) to convert X-Rays to visible light. Traditionally, the light generated by the scintillator is imaged onto a CCD sensor using a bulky lens system. In modern flat-panel detectors, the scintillator output is captured by either an amorphous silicon (TFT) panel or CMOS sensor that converts the image to digital format.

Medical Radiography

XRay Image Processing TechnologiesSeveral digital X-ray technologies exist, including computed radiography (CR), direct conversion and indirect detection. CR uses imaging plates consisting of scintillators and phosphors for exposure and reads the plates in a separate scanning operation (much like processing film); CR therefore cannot be used for real-time X-ray imaging such as cardio-vascular examinations. Direct conversion uses materials such as amorphous selenium to detect x-ray photons directly for immediate image output. Indirect detection uses scintillators (such as gadolinium oxides or cesium iodides) which convert x-rays to visible light, coupled to CCD or CMOS imagers to deliver high-quality, detail-rich images.

Each technology has tradeoffs in performance and cost, but regardless of technology X-ray detectors are judged by characteristics including sensitivity, noise, image contrast and resolution, as well as operational costs, stability and durability. In medical applications perhaps the most important X-ray detector imaging performance characteristic is detective quantum efficiency (DQE). High DQE (efficient use of incoming x-rays) is vital to minimizing patient dose and obtaining the best starting image to support further medical image processing and diagnosis.

CMOS Imaging Technology for X-Ray Imaging

CMOS image detectors offer numerous advantages including the ability to record smaller image details at higher resolutions – allowing for the diagnostics of medical anomalies at earlier stages, and significantly increasing the probability of early intervention, patient recovery and reduced treatment costs. Current and planned wafer-scale X-Ray CMOS image sensor designs are using pixel pitches ranging from 20 to 100 microns.

Another important benefit of CMOS image sensors is the absence of so-called ‘image lag’, or the presence of residual image information in successive images, that is typical of amorphous TFT technologies. Image lag leads to poor quality images because of small image displacement.